An ohmmeter is an instrument which measures and displays the value of resistance connected across its input terminals, but in most cases it is either impossible or inconvenient to directly connect the element or circuit under test to these input terminals. Consequently, long flexible leads are generally used to connect the input terminals of the meter to the element or circuit whose resistance is to be measured. Since, however, the leads themselves have a significant resistance value, unless compensation for the lead resistance is made, the resistance value displayed by the meter will be erroroneous, i.e., it will be the sum of the lead resistance and the resistance intended to measured.
In order to eliminate the resistance of the leads from the measurement being made, the common practice has been to provide the ohmmeter with means for adjusting the resistance value being displayed. In practice, the technician, while holding the two leads in mutual contact so that the lead resistance is connected across the input terminals, adjusts the meter to display a value of zero. Thereafter, the resistance of the leads will be automatically subtracted from subsequent measurements. This prior art method of compensating for lead resistance has two principal disadvantages. One such disadvantage arises from the fact that the zero adjustment must be repeated each time the resolution or range of the meter is changed. Consequently, that method of compensating for lead resistance cannot be used with meters having automatic ranging. The other principal disadvantage is that the adjustment is time consuming, and errors of adjustment easily occur.